This invention relates to thin film electroluminescent devices. More particularly, it is concerned with methods of depositing the phosphor layers of thin film electroluminescent devices by sputtering.
Thin film electroluminescent devices are employed for various forms of displays. Typically the devices employ a transparent substrate having on one surface a very thin conductive electrode which is substantially transparent. This first electrode is covered with an insulating layer. A layer of a suitable phosphor material overlies the insulating layer. The phosphor layer is covered with another insulating layer, and a second conductive electrode of an appropriate pattern is formed on the second insulating layer. Under operating conditions a voltage is applied between the two electrodes causing the portion of the phosphor layer between the electrodes to luminesce, thus providing a visible pattern when viewed through the transparent substrate.
Various methods have been used to deposit the layers constituting thin film electroluminescent devices including thermal evaporation, electron beam evaporation, chemical vapor deposition, and sputtering. From the point of view of an ecomonical manufacturing process, it is desirable that all the layers be deposited by sputtering. There is no problem as to the conductive and insulating layers since sputter deposition techniques for the materials typically employed for these layers are well known. Heretofore, however, it has been difficult to obtain phosphor layers of satisfactory characteristics by employing sputtering techniques. In order to obtain satisfactory phosphor layers they have usually been formed by employing evaporation methods, which are less easily adapted to large-scale production than is sputtering and, therefore, are more expensive.
Typically the phosphor layer is a host of zinc sulfide (ZnS) containing an activator, frequently manganese (Mn). In forming zinc sulfide phosphor layers by sputtering, the conventional technique includes applying rf energy to a target of zinc sulfide enclosed in a chamber containing a sputtering atmosphere of argon and a small quantity of hydrogen sulfide. One of the disadvantages of this process in producing phosphor layers in electroluminescent devices is that during the target erosion process very small particles of zinc sulfide tend to form. Many of these particles impinge on the substrate and become incorporated into the growing film where they represent discontinuities in both the geometric shape and crystal structure of the otherwise highly oriented phosphor film. In addition after exposure of a zinc sulfide target to air, a long period of presputtering is necessary in order to return it to a safisfactory condition for use as a sputtering target.